The same explanation is here:
X-X-X: Pokemon line ups used to describe basic, stage 1 and stage 2 evolution line.
Example: "I run a 4-2-4 of Charizard. So 4 Charmander, 2 Charmeleon, 4 Charizard"
"I run a 2-2 Blissy line. So 2 Chansey and 2 Blissy"